Vapor deposition processes are known for their use in the field of semiconductor devices. For example, metal organic chemical vapor deposition (MOCVD) has become an important process in the fabrication of semiconductor devices. MOCVD is a gaseous crystal growth technique in which compounds, e.g. trimethyl gallium ((CH.sub.3).sub.3 Ga), are caused to react with other gases, e.g. arsine (AsH.sub.3) and/or phosphine (PH.sub.3), and appropriate dopants, in the vapor phase to produce single crystalline or polycrystalline deposits.
For many applications MOCVD is favored over traditional liquid phase epitaxy (LPE) techniques because MOCVD provides better control over the layer thickness, layer smoothness, and the abruptness of layer interfaces. The MOCVD process is further characterized by greater uniformity of crystal composition and doping profiles as compared to LPE.
Typically an MOCVD system comprises a reaction chamber integral with a linear series of valves for introducing the appropriate reagents into the chamber. A problem with such a system is that the valves are a distance from the substrate to be coated resulting in an inherent lag time during the switching of reagents while terminating the growth of a first layer and commencing the growth of a second. Also, some reagents are introduced by valves at a greater distance from the substrate than others which can cause compositional variations at the beginning and end of each layer. With the increasing need to fabricate devices containing thinner layers, these problems become more pronounced. Therefore, an improved MOCVD apparatus has been sought.